Asteroid vs comet -
what's the difference?

Asteroids are predominantly rocky and metallic. The rocky types are silicate-rich; the metallic types are fragments of the iron-nickel cores of bodies that melted and differentiated early in solar system history. Neither type contains significant amounts of ice.

Comets have an icy nucleus - a mixture of water ice, frozen carbon dioxide, carbon monoxide, methane, dust, and complex organic compounds. The nucleus is typically between 1 and 50 kilometres across, dark in colour, and loosely structured. When a comet approaches the Sun and surface temperatures rise, ices begin to sublimate directly from solid to gas. This vaporisation produces the visible coma - a diffuse, fuzzy atmosphere around the nucleus that can extend tens of thousands of kilometres - and the characteristic tails.

The activity level depends on how much of the surface is exposed ice and how close the comet approaches the Sun. Comets that have made many previous passes become progressively less active as surface ices deplete.

Asteroids formed in the inner solar system during its first few million years. Most are confined to the main asteroid belt between Mars and Jupiter. A subset - the near-Earth asteroids (NEAs) - have been nudged into Earth-crossing orbits by gravitational interactions, primarily with Jupiter.

Comets originate from two reservoirs. Short-period comets (orbital periods under roughly 200 years) come from the Kuiper Belt, a disc-shaped region of icy bodies extending beyond Neptune's orbit from about 30 to 50 AU. One AU is approximately 150 million kilometres - the average Earth-Sun distance. The Kuiper Belt is where Pluto lives. Gravitational interactions with the outer planets occasionally send Kuiper Belt objects on inward trajectories.

Long-period comets originate from the Oort Cloud, a distant, roughly spherical shell surrounding the solar system that may extend up to 100,000 AU from the Sun - nearly a light-year away. Objects there are perturbed inward by passing stars or galactic gravitational effects. These comets may be on their first-ever pass through the inner solar system, arriving on orbits that take hundreds of thousands or even millions of years to complete.

As a comet warms, sublimating ice lifts dust and gas off the nucleus surface. This material expands outward and forms the coma - a loosely held atmosphere that can be larger than the planet Jupiter. The nucleus itself remains tiny compared to the surrounding cloud.

Two distinct tails develop from the coma. The dust tail is composed of solid particles pushed back by solar radiation pressure. It appears yellowish (reflecting sunlight), arcs gently along the comet's orbital path, and can stretch millions of kilometres. The ion tail - also called the plasma tail - consists of ionised gas stripped from the coma and swept directly away from the Sun by the solar wind. It appears bluish and points precisely away from the Sun, completely straight.

Both tails always point away from the Sun, regardless of whether the comet is approaching or receding. When a comet moves away from the Sun after perihelion - its closest approach - it travels tail-first. The tails lead, not trail.

Around 100 known comets qualify as near-Earth objects (NEOs) - bodies with a perihelion distance of less than 1.3 AU from the Sun. NEOs include both asteroids and comets; the distinction is compositional, not orbital.

Near-Earth comets present a different tracking challenge to asteroids. Their orbits are typically more eccentric - highly elongated ellipses - and outgassing creates non-gravitational forces that can nudge the orbit in ways that gravitational models alone cannot predict. This makes long-range orbital forecasting more uncertain for comets than for inert rocky bodies. NASA's Center for Near Earth Object Studies (CNEOS) monitors them alongside asteroids using the same automated systems.

Potentially Hazardous Asteroids, or PHAs, are defined as objects 140 metres or larger that come within 0.05 AU of Earth's orbit. A small number of near-Earth comets meet this threshold too - though the term is most commonly applied to the rocky population.

Some objects resist easy classification. Active asteroids have orbits indistinguishable from typical asteroids but show cometary features - a visible coma, a dust tail, or both. They may contain buried ice exposed by an impact or rotational disruption, or they may be releasing dust through some other mechanism.

3552 Don Quixote is the most striking example. Classified as an asteroid for decades after its 1983 discovery, it was later found to exhibit a faint coma and tail when close to the Sun. It is now thought to be a dormant or extinct comet nucleus - a body that once showed full cometary activity but has depleted most of its accessible surface ice.

Damocloids are another borderline group: asteroid-designated objects with highly eccentric, comet-like orbits but no observed activity. They are probably dormant or extinct comet nuclei whose surfaces have been sealed by a non-volatile crust, preventing sublimation even when close to the Sun.